On Sat, 19 Jan 2002, [EUC-KR] ¾È±â¿µ wrote:

...

I prefer Haskell style type classes because using them one can overload functions. e.g.

\begin{code} class Container a where celem :: Eq b => b -> a b -> Bool cmap :: (Eq b, Eq c) => (b->c) -> a b -> a c

data MyList a = MyList [a] deriving Show

instance Container MyList where celem x (MyList l) = elem x l cmap f (MyList l) = MyList (map f l) \end{code}

What does ML struct/functor have anything better than type classes ? For the user type classes feels like implicit functor istantiations to specific sturucture, and struct/functor seems just bugglling the user to do more typing which can be automated by using type classes.

Type classes are more than automatic functor instantiation since it can construct hierarcy between types and you can implement implicit type conversoins.

I think ML, especially nML - the Korean dialact of ML being developed in ROPAS(http://ropas.kaist.ac.kr/) - should throw away struct/functor or deprecate them and move to type classes.

p.s. Am I asking ML to metamorph into eager Haskell ? :-)

Automation comes with a stiff price: at most one implementation for each interface. In the above case, if I have a different implementation for MyList, how could it possibly be automatically determined which should be chosen? \begin{code} instance Container MyList where (a different implementation) \end{code} In Haskell, I guess that the one implemented later is always chosen. Why can't I have two different implementations for an interface? Another problem with Haskell classes is that there is currently no way of hiding type information. For instance, suppose that I want to implement a module for operations on sets but I do not want to reveal what data representation I use for sets. Is there a way of doing this in Haskell? --Hongwei \~~~~/ \\ // \\ // @ C-o^o, ))__|| \\__//_ // \\ ( ^ ) ))__|| \--/-\\ \\ / \V\ )) || // \\ \\ ------ // || o // \\ \\// Computer Science Department Boston University 111 Cummington Street Boston, MA 02215 Email: hwxi@cs.bu.edu Url: http://www.cs.bu.edu/~hwxi Tel: +1 617 358 2511 (office) Fax: +1 617 353 6457 (department)